This invention relates to line printers and, more particularly, to reverse polarity differential encoding for fringe field responsive electro-optic line printers.
It has been shown that an electro-optic element having a plurality of individually addressable electrodes can be used as a multi-gate light valve for line printing. See, for example, a copending and commonly assigned United States Pat. No. 4,281,904 of R. A. Sprague et al., which issued Aug. 4, 1981, now U.S. Pat. No. 4,281,904 on a "TIR Electro-Optic Modulator with Individually Addressed Electrodes," and a copending an commonly assigned United States patent application of R. A. Sprague, which was filed Sept. 17, 1980 under Ser. No. 187,911 on an "Electro-Optic Line Printer". Also see "Light Gates Give Data Recorder Improved Hardcopy Resolution," Electronic Design, July 19, 1979, pp. 31-32; "Polarizing Filters Plot Analog Waveforms," Machine Design, Vol. 51, No. 17, July 26, 1979, p. 62; and "Data Recorder Eliminates Problem of Linearity," Design News, Feb. 4, 1980, pp. 56-57.
Almost any optically transparent electro-optical material can be used as the electro-optic element of such a light valve. As of now the most promising materials appear to be LiNbO.sub.3 and LiTaO.sub.3, but there are other materials which qualify for consideration, including BSN, KDP, KD.sup.x P, Ba.sub.2 NaNb.sub.5 O.sub.15 and PLZT. In any event, the electrodes of such a light valve are intimately coupled to the electro-optic element and are distributed in non-overlapping relationship widthwise of the electro-optic element (i.e., orthogonally relative to its optical axis), typically on equidistantly separated centers so that there is a generally uniform inter-electrode gap spacing.
To perform line printing with a multi-gate light valve of the foregoing type, a photosensitive recording medium, such as a xerographic photoreceptor, is exposed in an image configuration as it advances in a cross line direction (i.e., a line pitch direction) relative to the light valve. More particularly, to carry out the exposure process, a sheet-like collimated light beam is transmitted through the electro-optic element of the light valve, either along its optical axis for straight through transmission or at a slight angle relative to that axis for total internal reflection. Furthermore, successive sets of digital bits or analog signal samples (hereinafter collectively referred to as "data samples"), which represent respective collections of picture elements or pixels for successive lines of the image, are sequentially applied to the electrodes. As a result, localized electric bulk or fringe fields are created within the electro-optic element in the immediate vicinity of any electrodes to which non-reference level data samples are applied. These fields, in turn, cause localized variations in the refractive index of the electro-optic element within an interaction region (i.e., a light beam illuminated region of the electro-optic element which is subject to being penetrated by the electric fields). Thus, the phase front or polarization of the light beam is modulated (hereinafter generically referred to as p-modulation) in accordance with the data samples applied to the electrodes as the light beam passes through the interaction region. P-sensitive readout optics are used to convert the phase front or polarization modulation of the light beam into a correspondingly modulated intensity profile. For example, if the phase front of the light beam is modulated, Schlieren central dark field or central bright field imaging optics are used to image the modulated light beam onto the recording medium. Alternatively, if the input light beam is polarized, the polarization to intensity modulation conversion process may be performed by passing the polarization modulated output beam through a polarization analyizer prior to imaging it on the recording medium. "P-sensitive optics" is, of course, another coined term which is used herein to generically refer to optics for performing a phase front modulation or polarization modulation to intensity profile modulation conversion process on the light beam.
It has been found that the number of electrodes required to enable a fringe field responsive electro-optic line printer to achieve a given resolution can be reduced by a factor of just slightly less than two if the input data is differentially encoded. Each data sample of a differentially encoded data stream, other than the first, has a magnitude whose difference from the previous differentially encoded data sample corresponds to the magnitude of a respective input data sample. The first sample in the differentially encoded data stream is referenced to a common potential, such as ground. See a copending and commonly assigned United States patent application of W. D. Turner et al., which was filed Sept. 17, 1980 under Ser. No. 187,916 on "Differential Encoding for Fringe Field Responsive Electro-Optic Line Printers".